page_title: Channichthyidae
section_title: Evolution
hierarchical_section_title: Channichthyidae / Evolution
caption_attribution_description: Chaenodraco wilsoni
context_page_description: The crocodile icefish or white-blooded fish comprise a family of notothenioid fishes found in the Southern Ocean around Antarctica. They are the only known vertebrates to lack hemoglobin in their blood as adults. Icefish populations are known to reside in the Atlantic and Indian sectors of the Southern Ocean as well as the continental shelf waters surrounding Antarctica. Water temperatures in these regions remain relatively stable, generally ranging from −1.8 to 2 °C. One icefish, Champsocephalus esox, is distributed north of the Antarctic Polar Frontal Zone. At least sixteen species of crocodile icefish are currently recognized, although eight additional species have been proposed for the icefish genus Channichthys.
context_section_description: The icefishes are considered a monophyletic group and likely descended from a sluggish demersal ancestor. The cold, well-mixed, oxygen-rich waters of the Southern Ocean provided an environment where a fish with a low metabolic rate could survive even without hemoglobin, albeit less efficiently. 
  It is not known when the icefish evolved. There are two main competing hypotheses. The first is that they are only about 6 million years old, appearing after the Southern Ocean cooled significantly. The second suggests that they are much older, 15-20 million years.
  Although the evolution of icefish is still disputed, it is widely accepted that the formation of the Antarctic Polar Frontal Zone (APFZ) and the Antarctic Circumpolar Current (ACC) marks the beginning of the evolution of Antarctic fishes. The ACC is an oceanic current that moves in a clockwise northeast direction, and can be up to 10,000 km (6,200 mi) wide. This current formed 25-22 million years ago, and thermally isolated the Southern Ocean by separating it from the warm subtropical gyres to the north.
  During the mid-Tertiary period, a species crash in the Southern Ocean opened up wide range of empty niches to colonize. Despite the hemoglobin-less mutants being less fit, the lack of competition allowed even the mutants to leave descendants that colonized empty habitats and evolved compensations for their mutations. Later, the periodic openings of fjords created habitats that were colonized by a few individuals. These conditions may have also allowed for the loss of myoglobin.
caption: Chaenodraco wilsoni
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